Hydrostatic transmissions are commonly used to transmit drive torque in a motor vehicle and include a variable displacement hydraulic pump driven by an input shaft. A hydraulic fluid is pumped from the hydraulic pump to a fixed displacement hydraulic motor for driving an output shaft. In these devices, there is no mechanical linkage between the input shaft which drives the hydraulic pump and the output shaft which is driven by the hydraulic motor.
Many hydraulic pumps of the variable displacement type have a rotating cylinder block with pistons axially movable therein. The displacement of the hydraulic pump is proportional to the stroke of the pistons within the cylinder block. Where the hydraulic pump is of the axial piston type, the pistons engage a tiltable swashplate to vary the stroke of the pistons. When the swashplate is perpendicular to the axis of the cylinder block, the swashplate is in a neutral or a zero displacement position and the hydraulic pump has no output. When the swashplate is tilted to a position away from the neutral position, hydraulic fluid is pumped to the motor and power thereby is transmitted from the input shaft to the output shaft.
In order to selectively prescribe the position of the swashplate, a displacement control commonly is provided in communication with a source of pressurized fluid for directing fluid to a servo mechanism which, in turn, adjusts the swashplate position. Displacement controls take many forms, but in most cases they allow an operator to manually select a desired swashplate position and the corresponding hydraulic pump displacement.
A conventional displacement control includes a fluid metering valve having an internal spool which is axially movable in response to a command input. The command input shifts the valve spool, as by a fluid pressure applied to opposite ends of the spool, whereby the valve ports fluid to an appropriate servo mechanism for positioning the swashplate. In order to close the servo control loop between the metering valve and the swashplate, a feedback device, such as a mechanical linkage, interconnects the swashplate with the valve spool. The feedback device is operative to convert the relative displacement of the swashplate and valve spool to a feedback force which opposes the differential pressure acting on the spool. When the feedback force increases to the point where the pressure differential effectively is balanced, the valve spool remains stationary and the swashplate achieves a steady-state operating position.
An elastic spring typically is used in the feedback linkage to convert the relative displacement of the swashplate and the valve spool into a force quantity. Based on the stiffness and initial compression of the spring, a preload force is developed in the spring which opposes the differential command pressure. The valve spool is not displaced until the pressure differential exceeds the spring preload. The range of differential pressure over which the valve spool is nonresponsive is referred to as the deadband operating range for the feedback device.
The limiting case, in which the feedback spring hypothetically has an infinite preload, can be analogized to the case where no spring is provided at all, that is, with the valve spool rigidly linked directly to the swashplate. In this situation, there is no value of differential pressure capable of displacing the valve spool so that the swashplate remains fixed. Conversely, an insufficiently preloaded spring results in a dynamically unstable situation in which small deviations in differential command pressure can undesirably drive the swashplate to full angle. The spring preload, as established by the spring stiffness and spring compression, thus play a significant role in defining the response characteristics of the feedback apparatus and the hydrostatic transmission.
Present spring feedback devices in the displacement controls of hydrostatic transmissions fail to provide the capability of simply prescribing a desired spring preload in an inexpensive and reliable manner. The present invention is directed toward overcoming the problems set forth above in a novel and useful way.